The present invention relates generally to control of a power transmission in a work vehicle such as an agricultural tractor. More particularly, the invention relates to an improved technique for selecting a desired gear ratio in such transmissions based on the ground speed of the vehicle.
In field of transmission systems for work vehicles, such as agricultural tractors, a great number of transmission configurations and control schemes have been proposed and are presently in use. Such transmissions typically include a collection of intermeshing gears either fixed to transmission shafts or rotating freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between an engine output shaft and a transmission output shaft to transmit engine torque at a desired speed to driven wheels of the vehicle. Control systems for commanding engagement of the clutches typically include electronic circuitry that responds to operator controls, such as an upshift/downshift lever, a forward/reverse lever and the like in the vehicle cab. The control system sends electric signals to hydraulic valves that channel pressurized fluid to the clutches. The control systems thus cause the clutches to engage and disengage in predetermined combinations to smoothly accelerate, decelerate and drive the vehicle as desired by the operator. A transmission and a control system of this type are described in U.S. Pat. No. 4,425,620, issued on Jan. 10, 1984 and assigned to Steiger Tractor, Inc.
Among the many improvements proposed for control systems for work vehicle transmissions, a particularly useful feature is the ability of the control system to select and engage a gear ratio that matches the vehicle ground speed following a period of transmission disengagement. Such periods of disengagement typically occur when the vehicle is moving (i.e. being driven) and the operator depresses a clutch pedal, thereby placing the transmission in neutral while allowing the vehicle to coast. In known ground-speed-matching systems, the transmission controller monitors vehicle ground speed (or transmission output speed) and engine output speed, calculates the ratio of the speeds and selects the transmission gear ratio that most closely matches the speed ratio. When the operator subsequently releases the clutch pedal, the transmission controller engages the combination of clutches required to shift into the ground-speed-matched gear ratio. Transmission control systems employing ground-speed-matching routines of this type are described in U.S. Pat. No. 4,425,620, mentioned above, and U.S. Pat. No. 5,007,299, issued on Apr. 16, 1991 to Bulgrien.
While such ground-speed-matching routines provide a convenient transition between coasting or disengaged operation of the vehicle and subsequent driven operation, they are not without drawbacks. For example, such known transmission control systems generally shift the transmission into the ground-speed-matched gear ratio upon release of the clutch pedal, regardless of the actual magnitude of the ground speed or the speed range represented by the matched gear ratio. That is, whether the vehicle is travelling at relatively fast or slow ground speed, the ground-speed-matched gear ratio is engaged. Moreover, while known transmission control systems may permit an operator to set a maximum allowable gear ratio during certain phases of operation, such as in a constant ground speed mode, such systems generally do not permit an operator to command a gear ratio prior to transmission disengagement and to use the commanded ratio as a limit in the ground-speed-matching routine. As a result, a vehicle operator may inadvertently cause the vehicle to shift into a higher gear than he would have preferred simply by coasting with the clutch pedal depressed and subsequently releasing the clutch pedal, thereby invoking the ground-speed-matching routine.
Therefore, despite the convenience of known ground-speed-matching transmission control systems, there is a need for an improved system for selecting a ground-speed-matched gear ratio for work vehicle transmissions that avoids the drawbacks mentioned above. In particular, there is a need for a ground-speed-matching control system for such transmissions that takes into account the magnitude of the ground speed as well as relative level of the ground-speed-matched gear ratio in selecting the gear to be engaged upon invocation of a ground-speed-matching routine. Moreover, there is a need for a ground-speed-matching control system that selects the gear ratio to be engaged by taking into account the gear ratio selected by the vehicle operator prior to invocation of the matching routine.